For solids and pure liquids, you don't include them in the K expression—only gases and aqueous solutions. It’s because their concentrations are essentially constant. But the *real* reason I remember this is from my stupid, frantic pre-lab, writing "K = [Fe]..." for a rust experiment and my professor circling it, laughing, "Rajan, if solid iron counted, this constant would be the size of your delivery box!" That visual stuck.
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